T.A. Nijhuis

Enhancement of catalyst performance in the direct propene epoxidation: a study into gold-titanium synergy

Enhanced productivity toward propene oxide in the direct propene epoxidation with hydrogen and oxygen over gold nanoparticles supported on titanium‐grafted silica was achieved by adjusting the gold–titanium synergy. Highly isolated titanium sites were obtained by lowering the titanium loading grafted on silica. The tetrahedrally coordinated titanium sites were found to be favorable for attaining small gold nanoparticles and thus a high dispersion of gold. The improved productivity of propene oxide can be attributed to the increased amount of the interfacial AuTi sites. The active hydroperoxy intermediate is competitively consumed by epoxidation and hydrogenation at the AuTi interface. A higher propene concentration is favorable for a lower water formation rate and a higher formation rate of propene oxide. Propene hydrogenation, if occurring, can be switched off by a small amount of carbon monoxide.

Kinetic study of propylene epoxidation with H2 and O2 over Au/Ti–SiO2 in the explosive regime

A kinetic study of propene epoxidation with hydrogen and oxygen over a Au/Ti-SiO2 catalyst has been performed in a wide range of reactant concentrations including the explosive region in a micro reactor. The observed rate dependency on the reactants for the epoxidation and the competing direct water formation is discussed in relation to the current mechanistic insights in the literature. The formation rate of propene oxide is most dependent on the hydrogen concentration, in which the formation of an active peroxo species on the gold nanoparticles is the rate determining step. Deactivation is mainly caused by consecutive oxidation of propene oxide. Oxygen favours the regeneration of the deactivated catalytic sites. Water formation and propene epoxidation are strongly correlated. Water is formed via two routes: through the active peroxo intermediate responsible for epoxidation and from direct formation without involving this active intermediate. Improving the hydrogen efficiency should distinguish between these two routes of water formation. The active peroxo intermediate in epoxidation is competitively consumed by hydrogenation and epoxidation. The active gold site is blocked during deactivation.

Oxidative dehydrogenation of ethane to ethylene over phase-pure M1 MoVNbTeOx catalysts in a micro-channel reactor

Phase-pure M1 MoVNbTeOx catalyst plates have been prepared on a metal–ceramic complex substrate by a dip-coating method. At a temperature of 420 °C and atmospheric pressure, the performance of the M1-PVA catalyst plate in a micro-channel reactor approached an ethane conversion of ~60% and an ethylene selectivity of ~85% with a high catalyst productivity of 0.64 kgC2H4 kgcat−1 h−1. Due to the excellent heat transfer ability, it is demonstrated that the micro-channel reactor can achieve the same reactor productivity as a traditional fixed-bed reactor within only 20% of its volume. XRD, SEM and ICP characterization indicated that the M1-PVA catalyst plate has a high stability in the micro-channel system.

Carbon-coated ceramic membrane reactor for the production of hydrogen by aqueous-phase reforming of sorbitol

Hydrogen was produced by aqueous-phase reforming (APR) of sorbitol in a carbon-on-alumina tubular membrane reactor (4 nm pore size, 7 cm long, 3 mm internal diameter) that allows the hydrogen gas to permeate to the shell side, whereas the liquid remains in the tube side. The hydrophobic nature of the membrane serves to avoid water loss and to minimize the interaction between the ceramic support and water, thus reducing the risks of membrane degradation upon operation. The permeation of hydrogen is dominated by the diffusivity of the hydrogen in water. Thus, higher operation temperatures result in an increase of the flux of hydrogen. The differential pressure has a negative effect on the flux of hydrogen due to the presence of liquid in the larger pores. The membrane was suitable for use in APR, and yielded 2.5 times more hydrogen than a reference reactor (with no membrane). Removal of hydrogen through the membrane assists in the reaction by preventing its consumption in undesired reactions.

Metal-organic framework capillary microreactor for application in click chemistry

A Cu/PMA–MIL‐101(Cr) metal–organic‐framework‐coated microreactor has been applied in the 1,3‐dipolar cycloaddition of benzyl azide and phenylactetylene (click chemistry). The Cu/PMA–MIL‐101(Cr) catalyst was incorporated by using a washcoating method. The use of tetraethylorthosilicate (TEOS) and a copolymer pluronic F127 as binders resulted in a stable and uniform coating of 6 μm. The application of the Cu/PMA–MIL‐101(Cr) capillary microreactor in the click‐chemistry reaction resulted in a similar intrinsic activity as in the batch reactor, and a continuous production for more than 150 h time‐on‐stream could be achieved. The presence of water in the reagent feed led to reversible catalyst deactivation and was necessary to be removed to obtain a stable catalyst operation.

Selective production of methane from aqueous biocarbohydrate streams over a mixture of Platinum and Ruthenium catalysts

A one-step process for the selective production of methane from low-value aqueous carbohydrate streams is proposed. Sorbitol, used herein as a model compound, is fully converted to methane, CO2 , and a minor amount of H2 by using a physical mixture of Pt and Ru (1:5 in mass basis) at 220 °C and 35 bar. This conversion is the result of hydrogenolysis of part of the sorbitol over Ru and the in situ production of H2 through the aqueous-phase reforming of the remaining carbohydrate over Pt. A synergistic effect of the combination of these two catalysts results in the rapid and highly selective conversion of the carbohydrate to methane. This process offers the possibility of upgrading a low-value carbohydrate stream into a valuable fuel with no addition of H2. Exergy analysis reveals that nearly 80 % of the exergy of the reactant is recovered as methane.